feat: Implement a new links menu, consolidate the main action keyboard into a ui module, and update employee ID generation logic.

modules/ui.py

@@ -0,0 +1,13 @@
+from telegram import ReplyKeyboardMarkup
+
+
+def main_actions_keyboard() -> ReplyKeyboardMarkup:
+    """Teclado inferior con comandos directos (un toque lanza el flujo)."""
+    return ReplyKeyboardMarkup(
+        [
+            ["/welcome"],
+            ["/vacaciones", "/permiso"],
+            ["/links", "/start"],
+        ],
+        resize_keyboard=True,
+    )